1. Field of the Invention
The present invention relates to a light emitting element comprising diamond and a method for producing the same. More particularly, it relates to a light emitting element comprising diamond containing N-V color centers and a method for producing the same from artificial diamond. The light emitting element comprising diamond is used as an element of a solid state laser which is tunable in a visible region.
2. Description of the Prior Arts
As a laser which is tunable in the visible region, a dye laser is known and used. But, the dye laser has several drawbacks. For example:
1. Since the dye is deteriorated by excitation light, output is unstable.
2. Since the dye is used in the form of a solution, a concentration of emission centers is low so that high output is not achieved.
3. Handling of the dye laser is troublesome.
Thus, it has been desired to provide a tunable solid state laser.
As a solid state laser, an alexandrite laser is known. This laser continuously emits light in a wavelength range of 701 to 826 nm (nanometers) at several ten watts, and finds applications such as processing or lithography. However, the alexandrite laser emits light having comparatively long wavelength.
Diamond is one of the most promising material for the solid state laser Possibility of application of diamond in a laser was described in Diamond Research, 1977, 11-14; Diamond Research, 1977, 15-23; Reports on Progress in Physics, 42, 1605-1659 (1979); and J. Phys. C: Solid. 16 (1983) 5417-5425.
The pure diamond has a large band gap of 5.5 eV. If it does not contain any impurity, it does not absorb or emit any light up to ultraviolet wavelength range. Therefore, color centers are to be created in the diamond crystal. To create the color centers in the diamond, the nitrogen atoms contained in the diamond are converted to one or more of following four types:
1. Ib type (discrete dispersion type) PA1 2. IaA type (two nitrogen atoms aggregate) PA1 3. IaB type (four nitrogen atoms aggregate) PA1 4. Accumulated type PA1 1. N-V color center (Ib type nitrogen-vacancy) PA1 2. H3 color center (IaA type nitrogen-vacancy) PA1 3. H4 color center (IaB type nitrogen-vacancy)
When each of these type nitrogen atom is combined with a vacancy, the following color center is created:
(wavelength of emitted light: 638-780 nm) PA2 (wavelength of emitted light: 503-600 nm) PA2 (wavelength of emitted light: 494-580)
The number of kinds of the color centers which can be created in the diamond may be about 60 in addition to the above three.
The application of color center in the laser was first reported by Dr. Stephen C. Rand in Optic Letters, 10 (1985) 481-483, in which pulse emission at 530 nm by using H3 centers was reported. However, laser emission by N-V centers has not been reported.
As described above, the N-V center is formed by combining the Ib type nitrogen atom with the vacancy (lattice defect). The N-V center absorbs excitation light in a wavelength range of 500 to 638 nm and emits luminescence in a wavelength range of 638 to 780 nm.
To form the N-V center in the diamond, a rough diamond containing Ib type nitrogen atoms is irradiated by an electron beam or a neutron beam to generate lattice defects. Then, the irradiated diamond is annealed by heating in vacuo to combine the lattice defect with the nitrogen atom to form the N-V center. The laser emission by the N-V centers is discussed in "Artificial Diamond for Color Center Laser", Solid State Lasers (Springer-Verlag) (1986). In this literature, the laser emission due to the N-V centers is said to be disadvantageous because of presence of quasistable level.
The solid state laser using diamond has still some disadvantages. For example, when natural diamond is used, a concentration of nitrogen atoms which contribute to the formation of color centers greatly varies from part to part, so that it is difficult to produce light emitting elements having the same quality. Since the natural diamond contains nitrogen atoms in three states, namely a IaA type, a IaB type and a discrete dispersion type, different color centers are formed corresponding to these states so that the element contains plural kinds of color centers, which results in unsatisfactory emission. When artificial Ib diamond produced by the thin film growing method is used, a relaxation time is short and intensity of emitted light is weak.